L-fluoro-2.4-dinitrobenzene

The results obtained with methoxypyrazole (247) are attributed to an electronic effect, whereas those obtained with (246) can be explained by taking into account the angular strain in the minor isomer. Reaction of 3(5)-nitropyrazole with l-fluoro-2,4-dinitrobenzene affords exclusively l-(2,4-dinitrophenyl)-3-nitropyrazole (70JHC1237). 

Concerning the reaction rate, a considerable decrease is observed qualitatively when R = C02Et or Ph. The presence of two Bu groups in the 3- and 5-positions (242 r3 = r5 = gyt) completely inhibits the reaction. Structure (248) for the transition state has been established from a kinetic study of the reaction between pyrazole and l-fluoro-2,4-dinitrobenzene <72JCS(P2)1420). 

The reaction has been extended to indazole (67BSF2619) from which both isomers have been obtained, and to pyrazolones (68BSF5019). In the latter system N-, O- and C-aryl and even diaryl derivatives have been isolated from the reaction with l-fluoro-2,4-dinitrobenzene. 

Several chemical methods have been devised for identifying the N-tenninal anino acid. They all take advantage of the fact that the N-tenninal anino group is free and can act as a nucleophile. The a-anino groups of all the other anino acids are part of amide linkages, are not free, and are much less nucleophilic. Sanger s method for N-tenninal residue analysis involves treating a peptide with l-fluoro-2,4-dinitrobenzene, which is very reactive toward nucleophilic aromatic substitution (Chapter 23). 

Reaction of the B chain peptide with l-fluoro-2,4-dinitrobenzene established that phenylalanine is the N terminus. 

The reaction is caiiied out by mixing the peptide and l-fluoro-2,4-dinitrobenzene in the presence of a weak base such as sodium car bonate. In the fir st step the base abstracts a proton from the terminal H3N group to give a free anino function. The nucleophilic fflnino group attacks l-fluoro-2,4-dinitrobenzene, displacing fluoride. 

When Sanger s method for N-tenninal residue analysis was discussed, you may have wondered why it was not done sequentially. Simply start at the N terminus and work steadily back to the C terminus identifying one anino acid after another. The idea is fine, but it just doesn t work well in practice, at least with l-fluoro-2,4-dinitrobenzene. 

Sanger s reagent (Section 27.11) Thecompound l-fluoro-2,4-dinitrobenzene, used in N-terminal amino acid identification. 

Buyuktimkin and Buyuktimkin [20] described a spectrophotometric method of assay of penicillamine and its tablets. An aqueous solution of 100 mg/mL of penicillamine was added to ethanolic 5 mM Sanger reagent (l-fluoro-2,4-dinitrobenzene), solid NaHC03, and water. The solution was diluted and heated at 70 °C for 45 min. After cooling, the solution was diluted with 3% ethanolic HC1. The absorbance of the resulting yellow complex was measured at 355 nm (molar absorptivity = 19,721). Recovery of the drug from commercial tablets was 99.1 0.7%. 

Bilikova and Kuthan [87] developed a gas chromatographic method for the determination of submicrogram concentrations of Carbofuran (2,3-dihydro-2, 2,-dimethylbenzofuran-7-yl-methyl carbamate) in soils. Soil samples are mixed with methanol-water (80 20) and water samples are extracted with chloroform. After separation of the chloroform, and weak alkaline hydrolysis, derivatization is performed with l-fluoro-2,4-dinitrobenzene. The ester produced is isolated from benzene and cleaned up with acetone. The acetone extract is used to determine Carbofuran by gas chromatography with a nitrogen-specific detector. 

TABLE 1. Apparent stability constant (Kc) of complexes between aromatic amines and l-fluoro-2,4-dinitrobenzene (unless otherwise indicated) at 40 °C... 

TABLE 1. Reaction of l-fluoro-2,4-dinitrobenzene (DNFB) with piperidine in aprotic solvents at 15°C48a second-order overall rate coefficients ... 

The study of molecular complexation was then extended to other aromatic nitro derivatives125. Although, as was described before, one of the more frequent methods of studying the formation of molecular complexes is by UV-visible spectrophotometry, the author did not observe detectable differences in the UV-visible absorbance spectra between the 2-hydroxypyridine-l-fluoro-2,4-dinitrobenzene (FDNB) mixtures and the sum of their separate components. The author observed that the signals of the 1II NMR spectra of FDNB in apolar solvents were shifted downward by the addition of 2-hydroxypyridine from solutions where [2-hydroxypyridine] [FDNB] he calculated the apparent stability constants, which are shown in Table 13. 

The authors presume that the observed effect is due to acid catalysis by methanol, but no catalysis by phenol was observed. Pietra and Vitali111 have shown earlier that phenol catalyses the reaction of l-fluoro-2,4-dinitrobenzene with piperidine in benzene. 

Support for this mechanism has been obtained from the study of the effect of twelve hydrogen-bond acceptors on the reactions of 1-chloro- and l-fluoro-2,4-dinitrobenzenes with morpholine in benzene162. The reaction of l-chloro-2,4-dinitrobenzene is not catalysed by either morpholine or DABCO, i.e. kA = h the first stage of the reaction is rate-determining and the various additives have no effect on the rate constant. On the other hand, Table 22 shows that the reaction of the fluoro-substrate is highly sensitive to the presence of the various additives and it is base-catalysed while for ten additives there was a linear dependence of kA on either their concentration, [P], or on the square... 

TABLE 22. Effect of some additives P on the reaction of l-fluoro-2,4-dinitrobenzene with morpholine in benzene at 30 °C. Values of k" (mol2 1 2 s 1) in the equation = k + k" [P] and other relevant data162... 

The reaction of the carbanion derived from diethyl methylphosphonate with perhalogenated aromatics may result in substitution of halide to yield perhaloaryl(hetaryl)methylphosphonates, which can be converted into tris- or bis-(perhaloaryl)methanes. Displacement of fluoride ion has been reported in the reaction of dimethoxycarbene with l-fluoro-2,4-dinitrobenzene and with hexafluorobenzene. The hydrodehalogenation of halogenated aryl ketones may be facilitated using hydrogen over a Pt/C catalyst. " ... 

The electrochemical behavior of l-fluoro-2,4-dinitrobenzene, l-chloro-2,4-dinitrobenzene, and l-bromo-2,4-dinitrobenzene in DMF was described (Gallardo et al. 2000) as follows. The 1-fluoro-2,4-dinitrobenzene anion-radical dimerizes before cleavage, whereas l-chloro-2,4-dinitrobenzene... 

Lysine is an essential amino acid with an e-amino group on the side chain that can react with various food components. As known, reaction of the e-amine can render lysine nutritionally unavailable reducing the nutritional value of food. While the determination of total lysine is straightforward (it is stable to acid hydrolysis), the determination of available lysine is difficult as lysine adducts are labile to the standard acid hydrolysis. A solution to this problem consists of derivatizing the e-amino group with a chromophore such as l-fluoro-2,4-dinitrobenzene (FDNB) to form a derivate which is stable to optimized hydrolysis conditions [222]. 

A kinetic flow injection analytical method was also described for isoxsuprine, which was based on the use of a fluoride-selective electrode for detection after reaction with l-fluoro-2,4-dinitrobenzene [38]. 

---